Conventional golf balls generally include a core surrounded by a cover. The cover forms a spherical outer surface of the ball and the surface includes a plurality of dimples. Typically the term “land” means the area of the outer surface of the ball not covered with dimples so that the land area is the outer surface of the ball between dimples.
Conventional dimples are circular depressions that act to reduce drag and increase lift. By using dimples to decrease drag and increase lift, golf ball flight distances have increased. The circumference of each dimple is the edge formed where the dimple wall slopes away from or intersects the land area of the outer surface. Since the geometry of the dimple principally determines ball drag and lift, conventional dimple patterns have been designed to optimize dimple geometry to reduce drag and increase lift.
Injection molding is a conventional method for forming the cover or an intermediate layer. According to well-known techniques, injection molding generally utilizes a mold and an injection unit. Referring to FIG. 1, a lower mold half 5 of a conventional injection mold is shown. The lower mold half fits into a bottom mold plate (not shown) and defines a hemispherical molding cavity 10 for receiving the core. The plate defines a runner system (not shown) for transporting a molten, cover material to one or more gates 15. The gates 15 allow the material to enter the cavity 10 from the runner system.
The mold also includes a plurality of separate, retractable pins 20; a vent pin (not shown) and a cluster block 25. The cluster block 25 defines bores for each pin 20 so that the pins extend therethrough and are affixed thereto. The pins 20 and the vent pin contact the core in generally the pole area of the core. Typically, the outer surface of the mold m cavity includes a plurality of hemispherical projections 30 for forming the majority of the cavity includes a plurality of hemispherical projections 30 for forming the majority of the dimples on the ball. The vent pin usually does not move and typically includes a free end shaped to form a dimple or land area depending on its location with respect to the dimple pattern being formed. An upper mold half and top plate of a similar configuration are also used.
One molding cycle for forming a golf ball includes a number of steps. When the top and bottom plates and lower and upper mold halves are separated, the core is placed within the bottom hemispherical molding cavity 10 on the pins 20, and the mold plates are closed to form a spherical cavity around the core. The pins center the core in the spherical cavity during molding. Then, the injection unit forces the molten, cover material through the runner system and gates into the molding cavity, until the cavity is filled and the material surrounds the core. The pins begin to retract as the material comes into close proximity to the pins. The material flows and fills the apertures in the material caused by the pins. As the material cools, it solidifies in the shape of the molding cavity around the core to form the golf ball. When the material is sufficiently cool, the mold plates and mold halves are again separated and the retractable pins are extended to separate the formed golf ball from the outer surface of the cavity also known as ejecting the ball from the mold. Then, mold is made ready for another molding cycle.
The retractable pins are located where a dimple or land area will be formed on the ball. If the retractable pins are located in dimple spaces, which are shaded areas 35 in FIG. 2, the free ends are substantially hemisphere-shaped (as shown in FIG. 1). In the retracted position during molding, each hemisphere-shaped, free end forms a single dimple in the outer surface of the ball. If the retractable pin is located in the land area, each such free end is shaped like the land area. As a result, in the retracted position during molding, these free ends form the associated, land areas. There are several drawbacks to these configurations.
Generally, golf balls have 300 to 500 circular dimples with a conventional sized dimple having a diameter that ranges from about 0.120 inches to about 0:180 inches. The retractable pins have similar dimensions at the free end to form the dimples. This leads to small surface areas at the free ends for each of the retractable pins. During ejection, since the free end surface area of each retractable pin is so small, the force each pin exerts on the ball is great. Accordingly, concentrated, high stresses are applied to the ball by the pins during ejection. These stresses can damage the ball in these areas so that extensive post-mold finishing, such as vibration tumbling, is done to make the balls playable. This is undesirable. In addition, the retractable pins slide with respect to the mold halves. This sliding forms “witness lines” about the pins in the retracted position. The clearance between the pins and the mold that causes these witness lines is about 0.0005 to 0.001 inches.
Another drawback is related to material flow during injection. When the material contacts the pins during molding, the pins are colder than the molten material. As a result, the molten material contacts the pins and begins to solidify about the pins, and the remaining molten material forms the cover. This also results in the formation of witness lines.
During retraction of the pins, when the material is packed around the pins, the pins can draw the material into the pin clearance between the pin and the mold. This material is often referred to as “flash material.” Flash material can also be formed when there is wear between the pins and the mold.
Post-mold finishing is conducted to remove the witness lines and flash material. Finishing to remove witness lines and flash on the dimple circumference can cause uncontrolled rounding of the dimple edges that can alter the flight characteristics of the ball undesirably. One way to reduce forming such material on the dimple circumference is to configure the pin so that the diameter of the pin is greater than the maximum dimple diameter. Such a mold is disclosed in Japanese Publication No. 61-213068. However, this mold has the drawback of requiring significant ejection force as discussed above.
An alternative to centering the core on pins throughout molding is disclosed in U.S. Pat. No. 3,068,552 entitled “Method and Apparatus for Molding Covers on Spherical Bodies” to Nickerson et al. This patent discloses a molding press with hemispherical cavities and a horizontally movable, retractable seat with an inner curved surface with the same radius of curvature as that of the wall of the cavity. The seat further includes a multiplicity of rectangular or rounded projections to form the checkered or dimpled outer surface on the completed golf ball. The patent further requires that the area of the curved surface of the seat be within certain limits; i.e., not more than 40% nor less than about 10% of that of the complete cavity wall and hence that of the spherical resilient, wound core. In the extended position, each retractable seat holds the core in an eccentric position. During molding, the patent discloses the core is moved from the eccentric position to the center of the mold. One drawback is that beginning in the eccentric position, it would be difficult to complete molding with a centered core. This would likely be partially due to gravity acting on the core during molding and moving the core downward. Another drawback is that no vent is shown in the seat, without this vent air would be trapped in the seat during molding and would create a void in the cover that is undesirable.
Consequently, a need exists for an improved molding apparatus and method for manufacturing a golf ball. The apparatus and method should decrease the likelihood of damaging the cover during ejection, and allow formation of the cover in such a way that post-mold finishing minimally changes the dimple circumference and requires less time than in conventional processes.